1. Field of the Invention
The present invention generally relates to an image forming apparatus using an electrophotographic developing process such as a laser beam printer, copier and facsimile machine, and more particularly, relates to a device for obtaining adjusted toner concentration of printed image in such an image forming apparatus.
2. Related Art
Electrophotographic developing process is widely used in computer printers, facsimile machines and photocopiers in order to produce images on recording media in response to video signals. A common example of an electrophotographic printing apparatus is a laser beam printer which prints images on individual sheet of paper through a series of electrostatic image-forming steps. Typically, the process of electrostatic image forming includes charging a photosensitive drum to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photosensitive drum is exposed to a light image of an original document being reproduced. This creates an electrostatic latent image corresponding to the document image on the photosensitive drum. After the electrostatic latent image is formed on the photosensitive drum, the latent image is developed by applying toner from a developing unit into contact with the latent image formed on the photosensitive drum. This forms a toner image of the original document on the photosensitive drum which is subsequently transferred and fixed on a recording medium. When a toner image is fixed on a recording medium, the toner image is first heated and fused onto the recording medium, and then naturally cooled so that it is fixed onto the recording medium.
Generally, as a unit for charging the surface of the photosensitive drum, a contact-type charging roller such as disclosed, for example, in U.S. Pat. No. 5,517,289 for Apparatus for And Method Of Forming Image issued to Ito et al., U.S. Pat. No. 5,479,243 for Image Forming Apparatus And Charging Device Thereof issued to Kurokawa, U.S. Pat. No. 5,247,328 for Method And Apparatus For Charging A Photoconductive Surface To A Uniform Potential issued to Daunton et al., and U.S. Pat. No. 5,164,779 for Image Forming Apparatus With Dual Voltage Supplies For Selectively Charging And Discharging An Image Bearing Member issued to Araya et al., using a so-called contact (or direct) charging scheme to produce an uniform electric field in response to application of high voltage for charging the surface of the photosensitive drum to a constant potential to attract toner particles and thereby form the latent image on the photosensitive drum. Typically, a charging unit is charged at the start of a printing operation. The surface of the photosensitive drum is charged and a developing unit is concurrently charged by way of the charging unit as the photosensitive drum rotates in a direction opposite to the rotation of the developing unit. The surface potential of a photosensitive drum is controlled by a number of known techniques as described, for example, in U.S. Pat. No. 5,072,258 for Method Of Controlling Surface Potential Of Photoconductive Element issued to Harada, U.S. Pat. No. 5,287,149 for Image Forming Apparatus Having Image Transfer Electrode Contactable To Transfer Material issued to Hoshika, and U.S. Pat. No. 5,534,982 for Developing Apparatus issued to Sakaizawa et al., in which the surface of the photosensitive drum is charged differently depending upon an image density and its background.
As a result, an electrostatic latent image is formed on the photosensitive drum and the latent image is then visualized as a toner image by the developing unit. The charged area of the photosensitive drum is then exposed to a laser beam. Because of the potential difference between the exposed portion and the unexposed portion of the photosensitive drum, the toner particles are attracted only to the exposed portion to form the toner image on the photosensitive drum, and its density may be adjusted without causing contamination to occur in the unexposed portion of the photosensitive drum as disclosed, for example, in U.S. Pat. No. 5,424,809 for Image Forming Method And Apparatus For The Same issued to Sawayama et al. After the toner image formed on the photosensitive drum is transferred to the recording medium, the photosensitive drum is charged back to an original voltage as the recording medium is being conveyed to a fixing unit. When the toner image is fixed on the recording medium by the fixing unit, the photosensitive drum is charged to a reference voltage. The residual toner on the surface of the photosensitive drum is subsequently cleaned by a cleaning blade of a cleaning device and is finally collected as waste toner. While contemporary surface potential controlling techniques contain their own merits, it is my observation that none can effectively eliminate unwarranted attraction of abnormal toner particles to the unexposed portion of the photosensitive drum which causes contamination to occur in the background of printed image on a recording medium.